The present invention relates broadly to the field of boiler-turbine controlled operations, and more particularly to an efficient valve position controller for governing the regulation of boiler throttle pressure to render the steam turbine admission valves in a selected one of a plurality of predetermined valve position states which correspond to valve operating points effecting minimum throttling losses.
It has been known for some time that the efficiency of a steam turbine power plant is degraded by the throttling losses that occur during the time when the steam admission valves of the steam turbine are governing steam flow in the partially opened state. It is understood that any improvement in efficiency of plant performance by reduction of these throttling losses will substantially reduce fuel consumption and provide a significant economic savings in the process of energy production. Various methods, such as: (1) constant throttle pressure-sequential valve operation; (2) throttling control-single valve operation; (3) sliding pressure; and (4) bypassing, have been utilized by some of the utilities to effect a reduction in valve throttling losses. For a more detailed description of these methods and how they compare to each other, refer to the paper entitled "A Review of Sliding Throttle Pressure For Fossil Fueled Steam-Turbine Generators" authored by G. S. Silvestri et al. which was presented at the American Power Conference, April 18-20, 1972. Conclusions of this paper indicate that "hybrid" type turbine designs which combine sequential valve and sliding throttle pressure operation, particularly the 50% admission "hybrid" units, have been shown to offer more efficient performance characteristics overall. The word "hybrid" was used in the Silvestri paper to describe boiler-turbine units that utilize constant throttle pressure-sequential valve operation down to some valve point, say 50% admission, at which time the valve position (admission arc) is held constant and the throttle pressure is reduced to attain lower flows. The Silvestri paper did not consider any method other than the "hybrid" method to further increase plant efficiency.
A similar "hybrid" type boiler-turbine plant operation has also been disclosed in U.S. Pat. No. 3,262,431 issued to F. J. Hanzalek on July 26, 1966. The Hanzalek patent is directed to an operation of sliding boiler pressure and sequential valve operation utilizing a particular boiler control configuration. It appears that Hanzalek's operation pertains to sliding boiler pressure during turbine start-up and initial loading to a value where optimum temperature and pressure conditions exist in the boiler and thereafter, increases in turbine steam flow are controlled by normal sequential valve movement at constant boiler pressure until another optimum boiler condition point is desired. In neither, the paper by Silvestri et al. nor the U.S. Pat. No. 3,262,431, is there described or even suggested any control system or method of improving plant efficiency by reducing throttling losses during the sequential valve mode steam flow governing operation periods.
Recently, improvements have been directed towards sequential valve control operation of turbine power plants by calculating a set of sequential valve position ranges which relate to minimizing throttling losses and providing an indication to the power plant operators when the steam admission valves have been sequentially positioned in one of these ranges. For a more detailed description reference is made to the copending application Ser. No. 628,629, referenced hereinabove. This improvement, of course, allows the power plant operator to select steam turbine operational points which correspond to minimizing throttling losses and provide a more efficient plant operation. On the other hand, this improvement normally consists of about 5 or 6 sequential valve position ranges of which each constitutes only approximately 3% or less of the steam flow; therefore, it is understood that the majority of sequential valve positioning is conducted at operational points which do not offer this minimizing effect with regard to throttling losses.
While there is a general awareness of the poor response with respect to operating turbine steam admission valves wide open and regulating boiler throttle pressure to govern load which is more commonly referred to as "sliding pressure" plant operation, some control system designers have continued to pursue this sliding pressure mode of operation by providing further improvement to the response thereof. One such control system is described in U.S. Pat. No. 3,802,189 issued Apr. 9, 1974 to T. W. Jenkins, Jr. Jenkins' system appears to provide a single point desired set point for a turbine control valve at a value preferably corresponding to a valve position near wide open. A rapid response to any increase in power generation demand is achieved by controlling the turbine control valve away from its steady state desired set point setting to a new position closer to wide open by a conventional turbine governor. As the actual valve position deviates from the desired set point value, the boiler throttle pressure set point is adjusted as a function of the position deviation to increase the boiler throttle pressure causing the power generation to increase beyond that demanded. Concurrently, the conventional turbine governor repositions the control valve until conditions exist which satisfy the requirements of the power generation being that demanded and the valve position being at the desired set point value. It appears that Jenkins' system controls power generation by sliding pressure in a boiler follow mode of operation permitting a faster response to power generation demand deviations as compared to a turbine follow mode of operation. However, it is understood that in order to achieve this improvement in response, Jenkins must relinguish some efficiency by steady state positioning the control valve away from a wide open position such that the turbine governor may be capable of responding quickly to power generation demand increases by modulating the control valve temporarily closer to a wide open position until the boiler throttle pressure can be readjusted. Thus, in Jenkins' system, it is believed that the control valve is inefficiently positioned during the majority of plant operation.
From the foregoing discussion, it appears that further improvements to boiler-turbine load control operations may be achieved in the areas of minimizing the throttling losses of the steam admission valves over a greater portion of the governing load range while at the same time maintaining an acceptable responsiveness of the steam turbine governor to changes in power generation demand.